In many geodetic applications, methods, devices and systems for measuring distances are used. According to the specific conditions in this field an accuracy or resolution below 1 cm is a typical requirement. However, for specific applications even further accuracy is necessary as in range finding and industrial distance measuring in some cases large distances have to be measured with sub-millimeter resolution.
Multiple-wavelength interferometry is, like classical interferometry, a coherent method, that offers the required accuracy but offers great flexibility in sensitivity by an appropriate choice of the different wavelengths. Interferometric measurement at different optical wavelengths enables the generation of new synthetic wavelengths, which are much longer than the optical wavelengths thereby allowing to increase the range of unambiguity and to reduce the sensitivity of classical interferometry. Moreover systems according to this principle can be operated on rough surfaces. The accuracy depends essentially on the properties of the source and on the signal processing.
In general, the source of a multiple-wavelength interferometer should produce an appropriate emission spectrum which comprises several discrete and stabilized wavelengths. In this case the range of non-ambiguity is given by the optical frequency difference. The stability and the calibration of the source will limit the absolute accuracy of the measurement. Moreover, the maximal distance which can be measured by multiple-wavelength interferometry is limited in prior art by the coherence length of the source. In addition, distance measurement on rough surfaces may be limited by the source power due to the scattering of the light. The design and the realization of the source are thus of a great importance, since the performance of the measuring set-up will be given by its properties, i.e. coherence, stability and power.
According to this principle the so called two-wavelength-interferometry (TWI) is a suitable technique for absolute distance measurement with a high resolution, as the use of two different wavelengths λ1 and λ2 creates a synthetic wavelength Λ=λ1λ2/|λ1−λ2| that is much greater than the optical wavelengths, thus increasing the non-ambiguity range. In order to obtain high accuracies over high distances, three requirements for suitable laser sources have to be fulfilled in prior art solutions.
First, the coherence length of the lasers has to be longer than twice the distance L between the target and the receiver. Secondly, the combination of synthetic wavelength and phase resolution has to be sufficient to perform the needed accuracy; thirdly, the synthetic wavelength has to be highly stabilised: for a relative uncertainty of distance δL/L=10−5 where δL is the resolution, the synthetic wavelength should be known with at least the same accuracy. Therefore, several techniques have been proposed in the prior art to fulfil the conditions: gas lasers have previously been used; however they are not suitable for compact systems.
In the prior art semiconductor laser diodes are discussed as the most energy efficient and the most compact lasers. Further, the emitted frequency can be tuned by changing the injection current and the temperature. Tunable lasers are of a great interest since the most appropriate synthetic wavelength can be chosen with more flexibility. However, when the most appropriate wavelength is chosen, they have to be frequency stabilized on an external reference.
Multimode laser diodes oscillate at a number of discrete wavelengths simultaneously, which provide a range of stable synthetic wavelengths if the laser is temperature controlled. The frequency separation between longitudinal modes is inversely related to the resonator length. The maximal synthetic wavelength which typically can be obtained in this way is therefore in the range of a few mm.
In standard single-mode AlGaAs diode lasers the light is confined in a semiconductor waveguide and the feedback is obtained by cleaving the crystal planes normal to the plane of the junction. They are known as Fabry-Pérot lasers. The line width is moderate (typically 10 MHz) and the frequency tunability with temperature is characterized by mode hops. These mode hops are mainly due to the temperature induced change of the centre of the gain curve (about 0.25 nm/° C.). The temperature tuning behaviour can vary from device to device. These discontinuities therefore limit the choice of synthetic wavelengths.
Distributed Bragg Reflector (DBR) diode lasers are devices where at least one of the cleaved facets is replaced by a Bragg grating. The Bragg grating acts as a frequency selective mirror. In distributed feedback (DFB) diode lasers the grating is manufactured along the active layer and act as a distributed selective reflector. The Bragg grating allows to increase the mode-hop free tuning range, since the tunability is mainly due to the temperature induced change of the refractive index. Moreover, the selective mirror leads to high side-mode suppression (>25 dB). This allows to substantially reducing the power independent contribution to the line width, which is mainly due to the mode partition noise in standard laser diodes. DBR and DFB laser diodes are thus suitable for multiple-wavelength interferometry. Tunable external cavity diode lasers may also provide a wide mode-hop free tuning range with small line width. For instance, a tuning range of at least 10 nm with less than 3 mode-hops can be obtained by using commercially available external cavity diode lasers (New Focus, Velocity Tunable Diode Laser). In addition, the line width may be less than 300 kHz. The main drawback is the complexity of the mechanical cavity. Tunable Nd:YAG lasers may also be of great interest for interferometry. The phase fluctuations and the line width of such lasers are smaller than for standard diode lasers. The frequency tunability is of about 50 GHz. However, Nd:YAG lasers exhibit poor efficiency since they require optical pumping by means of laser diodes.
For stabilization purposes atomic absorption lines are an option, but with limited choice of the synthetic wavelength and emission wavelengths. Another alternative are Fabry-Perot resonators wherein laser wavelengths are stabilised on the transmission peaks of the cavity. However, the stability is limited by the thermal expansion of the etalon length so that highly accurate measurements, e.g. δL/L<10−5, are impossible to achieve.
As already mentioned, Fabry-Pérot resonators are applicable to multiple-wavelength-interferometry, since the lasers can be stabilized on different resonances in order to generate a stable frequency difference and therefore a stable synthetic wavelength. However its stability is limited by the thermal expansion of the etalon length. This can be neglected if the resonator is made of super-invar or zerodur material for instance. The length of the Fabry-Pérot resonator may also be locked on a reference laser, e.g. a diode laser which is stabilized on an atomic absorption line. In this way, an absolute stabilization of every laser is achieved. This enables to combine multiple-wavelength interferometry with classical interferometry, by using one of these stable optical wavelengths to obtain an absolute distance measurement with submicrometer accuracy.
An algorithmic approach to overcome source side limitations is disclosed in WO2006/089864. To extend the coherence length limitation for the measurement distance a phase reconstruction algorithm is disclosed that allows the evaluation of interferometer signals without observable carrier signal. The measured phase response, i.e. signals from a quadrature receiver, is compared with a simultaneously measured reference signal.
With respect to detection and signal processing several approaches are discussed in the prior art. Heterodyne techniques allows to obtain a signal which is directly sensitive to the synthetic wavelength rather than to the optical wavelength. This is of a great importance, since interferometric stability at the optical wavelength is not any more required.
Superheterodyne detection, e.g. as disclosed in R. Dändliker, R. Thalmann and D. Prongué, “Two-wavelength laser interferometry using superheterodyne detection”, Proc. SPIE 813, 9-10 (1987) or R. Dändliker, R. Thalmann and D. Prongué, “Two-wavelength laser interferometry using superheterodyne detection”, Opt. Lett. 13, 339-341 (1988), enables high resolution measurements at arbitrary synthetic wavelengths without the need for interferometric stability at the optical wavelengths or separation of these wavelengths optically. Both wavelengths are used to illuminate simultaneously a Michelson interferometer. Two different heterodyne frequencies f1 and f2 are generated for each wavelength. These frequency differences can be produced by acousto-optical modulators and are typically f1=40.0 MHz and f2=40.1 MHz.
An overview of prior art is given in Y. Salvadé, “Distance measurement by multiple-wavelength interferometry”, Thesis, Institute of Microtechniques, Neuchâtel, 1999. Moreover, in this document a multiple-wavelength source with absolute calibration by opto-electronic beat-frequency measurement is disclosed. The three-wavelength source comprises three laser diodes operating at three different frequencies. Two of them are stabilized on two consecutive resonances of a common stable Fabry-Peŕot resonator used as frequency reference.